Short-wavelength lasers fabricated from Group III-nitride semiconductor materials, whose general formula is AlxGa1xe2x88x92xxe2x88x92yInyN, where Al is aluminum, Ga is gallium, In is indium, N is nitrogen, and x and y are compositional ratios, have been widely reported. However, such lasers have a far-field pattern, which is the Fourier transformation of the near-field pattern, that exhibits more than one peak. See, for example, D. Hofstetter et al., 70 APPL. PHYS. LETT., 1650 (1997). A laser that generates coherent light having a far-field pattern that exhibits multiple peaks can be used in substantially fewer practical applications than a laser that generates coherent light having a far-field pattern that exhibits a single peak.
The far-field pattern of the light generated by such conventional lasers exhibits multiple peaks, rather than the desired single peak, because the optical waveguide layer provides insufficient optical confinement and allows light to leak from the optical waveguide layer into the contact layer underlying the cladding layer. The contact layer then acts as a parasitic optical waveguide, resulting in spurious laser oscillation in a high-order mode. The contact layers are included in the laser to inject current into the active layer.
Attempts to achieve sufficient optical confinement have included increasing the thickness of the cladding layers compared with the conventional thickness value, and increasing the refractive index difference between the cladding layers and the optical waveguide layers. However, when implemented conventionally, these measures cause cracks in the cladding layers. This seriously impairs the production yield of lasers that incorporate such measures.
FIG. 1 illustrates the structure of the conventional GaN-based laser diode 10. The electrodes have been omitted to simplify the drawing. The laser diode is composed of the GaN low-temperature-deposited buffer layer 12, the GaN n-contact layer 13, the n-type AlGaN cladding layer 14, the n-type GaN optical waveguide layer 15, the active layer 16, the p-type GaN optical waveguide layer 17, the p-type AlGaN cladding layer 18, and the GaN p-contact layer 19. These layers are successively grown on the substrate 11. The material of the substrate is sapphire, SiC, spinel, MgO, GaAs, silicon, or some other suitable material.
The growth temperatures and growth thicknesses of the layers of a conventional laser diode having a structure similar to that shown in FIG. 1 are disclosed by Okumura in Japanese Laid-Open Patent Application No. H 10-261838. The low-temperature-deposited buffer layer 12 is a 35 nm-thick layer of GaN deposited at a temperature of 550xc2x0 C. The GaN n-contact layer 13 is a 3 xcexcm-thick layer of silicon-doped n-type GaN deposited at a temperature of 1050xc2x0 C. The n-type cladding layer 14 is a 700 nm-thick layer of silicon-doped Al0.1Ga0.9N deposited at a temperature of 1050xc2x0 C. The n-type optical waveguide layer 15 is a 50 nm-thick layer of silicon-doped GaN deposited at a temperature of 1050xc2x0 C. The active layer 16 is an 18 nm-thick composite layer deposited at a temperature of 750xc2x0 C. The composite layer is composed of three 2 nm-thick layers of In0.05Ga0.95N interleaved with four 3 nm-thick layers of In0.2Ga0.8N. The laser diode additionally includes an anti-evaporation layer (not shown), which is a 10 nm-thick layer of Al0.2Ga0.8N deposited at a temperature of 750xc2x0 C. The p-type optical waveguide layer 17 is a 50 nm-thick layer of magnesium-doped GaN deposited at a temperature of 1050xc2x0 C. The p-type cladding layer 18 is a 700 nm-thick layer of magnesium-doped Al0.1Ga0.9N deposited at a temperature of 1050xc2x0 C. The p-contact layer 19 is a 200 nm-thick layer of magnesium-doped p-type GaN deposited at a temperature of 1050xc2x0 C.
The above-mentioned layers are successively grown on the C plane of the sapphire substrate 11 by metal-organic vapor phase epitaxy (MOVPE). Alternatively, molecular beam epitaxy (MBE) or halide vapor phase epitaxy (HVPE) may be used.
After the above-described stack of layers has been deposited on the substrate, the stack of layers is annealed at 800xc2x0 C. in a nitrogen atmosphere to activate the dopants in the magnesium-doped p-type layers, and hence reduce the resistance of these layers. It has also been disclosed that the Al0.2Ga0.8N anti-evaporation layer (not shown) can be doped with magnesium, which facilitates hole injection from the p-type GaN optical waveguide layer 17.
Okumura reported that providing a ridge structure in the stack, depositing electrodes on the p-contact layer 19 and the n-contact layer 13 and cleaving the stack produced a laser diode that generated coherent light with a wavelength of 430 nm and had a threshold current of 40 mA.
Okumura""s disclosure does not indicate the far-field pattern of the light emitted by the laser just described.
In 37 JPN. APPL. PHYS., L905-L906 (1998), Yasuo Ohba et al. proposed a structure that uses a GaN active layer for generating light at short wavelengths. The GaN active layer required that the molar fraction of AlN in the AlGaN cladding layers be increased to maintain the band-gap difference between the active layer and the cladding layers. However, the increased aluminum molar fraction gave rise to a lattice mismatch between the materials of the AlGaN cladding layer and the GaN buffer layer on which it was deposited. The lattice mismatch was sufficiently large to cause cracks in the cladding layer.
Ohba et al. disclosed using a single-crystal AlN buffer layer interposed between the substrate and the n-type cladding layer to solve the cracking problem. First, a 600 nm-thick single-crystal AlN buffer layer was grown at a temperature of 1300xc2x0 C. on a sapphire substrate. Successively grown on the buffer layer were an n-type cladding layer, which was a 1.2 xcexcm-thick layer of silicon-doped n-type Al0.25Ga0.75N deposited at a temperature of 1150xc2x0 C.; an n-type optical waveguide layer, which was a 100 nm-thick layer of silicon-doped GaN deposited at a temperature of 1150xc2x0 C.; an active layer, which was a 50 nm-thick composite layer composed of one layer of Al0.2Ga0.8N, five layers of GaN interleaved with four layers of Al0.1Ga0.9N and one layer of Al0.2Ga0.8N; a p-type optical waveguide layer, which was a 100 nm-thick layer of magnesium-doped GaN grown at a temperature of 1150xc2x0 C.; a p-type cladding layer, which was a 700 nm-thick layer of magnesium-doped Al0.25Ga0.75N grown at a temperature of 1150xc2x0 C.; and p-contact layer, which was a 600 nm-thick layer of magnesium-doped GaN contact layer grown at a temperature of 1150xc2x0 C.
After the above-stack of layers was deposited, the stack was annealed in a nitrogen atmosphere at 800xc2x0 C. to activate the dopants in the magnesium doped p-type layers, and hence reduce the resistance of these layers. Ohba et al. reported that no cracking was observed even when the n-type cladding layer was grown on the single-crystal AlN buffer layer to an overall thickness of 1.8 xcexcm. According to Ohba et al., p-type AlGaN has a reasonably low resistivity at AlN molar fractions up to 25%. Ohba et al. further reported that, when electrodes were added to the device just described, the device emitted high-intensity light, but no coherent light was generated, even near the breakdown voltage of the device.
In Japanese Laid-Open Patent Application No. H 10-242587, Nagahama et al. stated: xe2x80x9cIn the case of an LD (laser diode), a cladding layer that provides optical confinement must be grown preferably with a thickness of at least 0.1 xcexcm, but if a thick AlGaN layer is grown directly on GaN and AlGaN layers, cracks will develop in the AlGaN layer that is subsequently grown. This has made device production difficult in the past.xe2x80x9d Nagahama et al. went on to disclose a technique that introduced a 10 to 500 nm-thick anti-cracking layer on which a layer can be grown thick enough for the subsequently-grown aluminum-containing layer to function as a cladding layer. Nagahama et al. stated that, while the anti-cracking layer can be left out under certain conditions, such as when certain growth conditions are used for growing the cladding layer, it should be included if an LD is being produced.
Nagahama et al. further disclosed an n-type cladding layer formed on the anti-cracking layer. The n-type cladding layer was a nitride semiconductor containing aluminum, preferably AlGaN. The thickness of the cladding layer was between 10 nm and 2 xcexcm, and was preferably between 50 nm and 1 xcexcm. In a working example, the n-type anti-cracking layer was an approximately 50 nm-thick layer of silicon-doped In0.1Ga0.9N grown at 800xc2x0 C., and the n-type cladding layer was a 500 nm-thick layer of silicon-doped Al0.2Ga0.8N grown at 1030xc2x0 C.
Nagahama et al. disclosed nothing about the far-field pattern of the coherent light emitted by a laser diode incorporating the structure just described.
In Japanese Laid-Open Patent Application No. H 10-256662, Ozaki et al. disclosed a cladding layer having a superlattice structure formed on an anti-cracking layer. In a working example, the n-type anti-cracking layer was an approximately 50 nm-thick layer of silicon-doped In0.1Ga0.9N grown at a temperature of 800xc2x0 C., and the n-type cladding layer was a 400 nm-thick superlattice multi-layer structure composed of 100 layers of silicon-doped GaN, each 2 nm thick, interleaved with 101 layers of silicon-doped Al0.2Ga0.8N. It was claimed that the crystal quality of the n-type cladding layer was extremely good because the thickness of the layers was below the critical thickness within the limit of elastic deformation.
Ozaki et al. disclosed nothing about the far-field pattern of the coherent light emitted by a laser diode incorporating the structure just described.
In Japanese Laid-Open Patent Application No. H 10-261816, Kuramata disclosed a technique in which a 1 xcexcm-thick layer of AlGaN was grown as a cladding layer directly on a substrate of 6H-SiC (0001) C. The material of the cladding layer was silicon-doped Al0.1Ga0.9N grown at 1200xc2x0 C. Kuramata additionally disclosed a structure in which an undoped 20 nm-thick layer of AlN and a 1 xcexcm-thick layer of n-type GaN were deposited at 1200xc2x0 C. as buffer layers on a SiC substrate, and a 200 nm-thick layer of silicon-doped Al0.1Ga0.9N was grown on the GaN layer as a cladding layer.
Kuramata disclosed nothing about the far-field pattern of the coherent light emitted by a laser diode incorporating the structure just described.
What is needed, therefore, is an efficient nitride semiconductor device, suitable for incorporation into a laser diode, that would permit the laser diode to generate coherent light having a far-field pattern that exhibits a single peak. Incorporating the nitride semiconductor device into a laser diode should enable the cladding layers of the laser diode to provide an increased optical confinement so that unintended light leakage from the optical waveguide region to the underlying contact layer is reduced. The optical confinement can be increased by increasing the thickness of the AlGaN material of the cladding layers, by increasing the molar fraction of AlN in the AlGaN of the cladding layers, or both. However, these measures have traditionally led to cracks in the cladding layers and, hence, poor device performance and poor production yields. The above-mentioned prior art provides no clear design guidelines as to how to enhance the optical waveguide provided by the cladding layers without the disadvantages just described.
The results reported by Ohba et al. indicate that cladding layers that are too thick, and incorporating an AlN buffer layer, may prevent a device having the structure of a laser diode from generating coherent light even though the layers do not have cracks. Also, since the lattice mismatch between AlGaN and GaN generates cracks and other defects, a thick, crack-free AlGaN cladding layer that provides sufficient confinement cannot be grown directly on a layer of GaN using conventional techniques. It is difficult to prevent a thick cladding layer of single-crystal AlGaN from cracking, even when the cladding layer is grown on an anti-cracking layer. Therefore, even when an anti-cracking layer is used, the AlGaN cladding layer have to have a superlattice structure, or other measures have to be taken. These measures, however, make the structure complicated, and lead to an unsatisfactory production yield.
What also is needed, therefore, is a nitride semiconductor device that can be incorporated into a semiconductor laser diode that generates short-wavelength coherent light having a far-field pattern that exhibits a single peak. The nitride semiconductor device should also have a simple structure, and the laser diode that incorporates the nitride semiconductor device should have a high efficiency, high reliability, and a long service life.
The invention provides a nitride semiconductor device that includes a first layer of a first material and a second layer of a single-crystal nitride semiconductor material that includes AlN. The second layer has a thickness greater than the thickness at which cracks would form if the second layer were grown directly on the first layer. Sandwiched between the first layer and the second layer is a buffer layer of a low-temperature-deposited nitride semiconductor material that includes AlN. The low-temperature-deposited nitride semiconductor material of the buffer layer is deposited at a temperature below that at which single-crystal growth occurs.
The nitride semiconductor device according to the invention, when incorporated into a laser diode, lowers the threshold current of the laser diode, and enables the laser diode to generate coherent light having a far-field pattern that exhibits a single peak. The nitride semiconductor device according to the invention enables the laser diode to include cladding layers that effectively confine light without being subject to cracking.
In the nitride semiconductor device according to the invention, the thickness of the second layer, or the AlN molar fraction of the single-crystal nitride semiconductor material of the second layer, or both, are greater than a value at which the coherent light emitted by the laser diode has a far-field pattern that exhibits a single peak.
The first layer on which the above-mentioned buffer layer is grown can be a layer of GaN or a substrate. The material of the substrate can be sapphire, SiC, silicon, MgAl2O4, GaN or some other suitable substrate material. The ability of the first layer to be a layer of GaN or a substrate enables one or more nitride semiconductor devices according to the invention to be incorporated into a semiconductor device at a location or locations where the elements can prevent cracking. Each nitride semiconductor device enables a thick, crack-free second layer of a material that includes AlN to be grown over a first layer of a material having a lattice constant different from the material of the second layer with the buffer layer sandwiched between them. Consequently, lasers and other semiconductor devices that incorporate the nitride semiconductor device according to the invention can have good performance characteristics and low manufacturing cost.
In an embodiment, the above-mentioned first layer of GaN is grown on a substrate on which has been deposited a buffer layer of low-temperature-deposited nitride semiconductor material. The buffer layer improves the surface quality of the first layer of GaN grown on it.
The buffer layer and the second layer may be doped with the same type of dopant. This reduces the resistance of the buffer layer and enhances the efficiency of the nitride semiconductor device.
To minimize the resistivity of the buffer layer, the doping concentration of the buffer layer of low-temperature-deposited nitride semiconductor material should be as high as possible, but less than that which degrades the crystal quality of the low-temperature-deposited nitride semiconductor material of the buffer layer.
Silicon, germanium, and the like can be used as n-type dopants, while magnesium, zinc, and beryllium can be used as p-type dopants. Silicon and magnesium have the effect of lowering resistance and allow technologically-mature fabrication methods to be used.
The thickness of the buffer layer should be at least that at which the buffer layer provides a stable buffering effect. However, the thickness should be no more than that at which a good crystal quality is maintained in the buffer layer itself and in the second layer grown on the buffer layer. Therefore, the thickness of the buffer layer should be in the range of 2 nm to 100 nm.
In an embodiment, the buffer layer has a thickness in the range from 20 to 40 nm, and the low-temperature deposited material that includes AlN is AlyGa1xe2x88x92yN (0 less than yxe2x89xa61). The second layer has a thickness of at least 600 nm, and the single-crystal nitride semiconductor material that includes AlN is AlxGa1xe2x88x92xN (0.05 less than xxe2x89xa61). With these materials and thicknesses, and when the molar fraction of AlN in the single-crystal nitride semiconductor material is at least 10%, a semiconductor laser incorporating the nitride semiconductor device according to the invention generates coherent light having a far-field pattern that exhibits a single-peak characteristic.
The molar fraction of AlN in the low-temperature-deposited nitride semiconductor material of the buffer layer is at least 5% to prevent cracking of the second layer. Increasing the AlN molar fraction increases the resistivity of the buffer layer. However, to prevent cracking, the AlN molar fraction in the low-temperature-deposited nitride semiconductor material should be the same as or greater than that of the single-crystal nitride semiconductor material of the second layer. Making the AlN molar fractions the same in both the buffer layer and the second layer is one choice.
Although other growth methods can be used, growing the buffer layer and the second layer using metal-organic vapor phase epitaxy produces a nitride semiconductor device that, when incorporated in a semiconductor laser, enables the laser to generate coherent light having a far-field pattern that exhibits a single peak.
The invention also provides a method of making a nitride semiconductor device. In the method, a first layer is provided, and a buffer layer of a nitride semiconductor material including AlN is deposited on the first layer at a temperature below that at which single-crystal growth occurs. A second layer of a single-crystal nitride semiconductor material including AlN is grown on the buffer layer at a temperature above that at which single-crystal growth occurs. The second layer is grown to a thickness greater than a thickness at which cracks would form if the second layer were grown directly on the first layer.